 In a multicellular organism, cells communicate with each other for various functions and also for the integration of various functions. The ways in which they communicate are varied and that we will discuss in the lecture on intercellular communication. So what is the significance in multicellular organisms? Signals actually determine whether a cell lives or dies. For example, there is a nerve suppose and it makes a contact with the muscle fiber. If there is damage to the neuron, what happens that muscle fiber also degenerates? How is it possible? This is possible because the contact between the neuron and the muscle is not only anatomical and not only for the functioning of the muscle but also for the growth of the muscle. So it releases certain trophic factors which are important for the survival of the muscle fibers. Similarly, there might be cells for which death is required. For example, they are infected with the virus. So these cells release chemicals outside bringing in the WVCs which cause death of that cell. So signals determine whether a cell lives or dies, whether it remains quiescent that is silent or performs a specific function. For example, the release of chemicals acts on receptors which are present in various cells in different different organs and depending on which chemicals are acting on the cells, cells perform a specific function. And they also determine whether a cell multiplies or differentiate or may even remain silent that it is not multiplying. So it may multiply in forming its own type cell or it may differentiate forming different lineages of the cell as we see in erythropoiesis depending on which chemical is acting on the cell. There might be different lineage differentiation. It may go into erythroid series or it may go into myeloid series. So what are the modes of intracellular communication? First it can be due to the release of chemical substances. So these chemicals can travel via blood to various cells of different organs or these chemicals may act on adjacent cells. Then it is due to the interaction of surface molecules on the cells. So there might be certain proteins which are present on the cells which interact with other tissues. Then there might be presence of communicating junctions between cells. So these are the three broad modes of intracellular communication. We will see each mode one by one. So first comes by the chemical substance and this signaling pathway is based on the spatial relationship between the sending and receiving cells. Sending means the cell which is releasing the chemicals and the receiving cell is the cell where the chemical is acting. And based on the spatial relationship that means how far or near the cells the sending and receiving cells are present in space, this signaling pathways are classified into endocrine pathway, paracrine, autocrine signaling and neurocrine or synaptic signaling. So what is endocrine signaling? Endocrine signaling is where the sending and receiving cells are present much far apart. So in this case if you see this diagram this is the sending cell and it is releasing the chemical in the blood and the chemical travels via blood and reaches to the receiving cells and there might be varied receiving cells present in different organs. So this is known as endocrine signaling or distant signaling. So the chemical is released in the blood. Then comes the paracrine signaling. In paracrine signaling the cells, sending cells and receiving cells are present adjacent to each other and there is release of the chemicals which act on the adjacent cells. So receptors for these chemicals are present on the adjacent cells. For example the release of histamine by intracromaphen like cells in stomach. This histamine is released locally and it acts on the parietal cells causing the stimulus for HCL secretion. So this paracrine signaling is basically adjacent signaling. Next comes the autocrine signaling. Autocrine signaling is important for self-regulation. So auto-hairword means that the chemical is released by the cell and it acts on the same cell. So the receptors for the chemicals are present on the same cell. After being the release of chemicals by T helper cells, so it releases certain chemicals which cause and acts on various other cells, cytotoxicity cells, B cells but it also releases chemicals which act on itself. Then in certain neurons, the release of the neurotransmitters, these neurotransmitters go and act on the post-synaptic neuron but these neurotransmitters can also act on the same neuron that is the pre-synaptic terminal from which it is released and regulate the release of the neurotransmitters from itself. Then finally there is neurocrine or synaptic signaling which is basically the release of the neurotransmitters by the neuron and these neurotransmitters acting on the post-synaptic membrane of another neuron. So we saw that based on the spatial relationship between the sending and receiving cells, there are four types of chemical signaling, chemical mode of intercellular communication. So first we have covered here, coming to second mode, interaction of surface molecules on the cells. Now the cells are connected to each other by means of intercellular junctions and these intercellular junctions are functionally classified as uploading junctions, anchoring junctions and communicating junctions. So the details of these junctions we are going to see in the next lecture but basically these two will be covered more in detail and communicating junctions we saw that it is also classified as third mode of intercellular communication so that we will see here itself. Now these intercellular junctions because of the presence of certain protein molecules and the attachment of these protein molecules to the cytoskeletal elements, they can also act as communication so it is not only for adhering the cell to a particular place or holding the cells together, it also changes the functions of the cell and this kind of signaling is known as juxtacrine signaling. So there is another term for this known as juxtacrine signaling. So how they interact as you see there are certain molecules there is integren okay so it is interacting with the extracellular matrix. Then one very good example is the platelet activation. In a platelet activation there is something known as inside out signaling and outside in signaling. So these are examples of these juxtacrine signaling. What happens that there are receptors on the platelets and these receptors interact with the extracellular matrix and there is change in the shape of the platelets which then causes a release of the chemicals. So these are examples of juxtacrine signaling. Let us come to the third type of signaling that is the communicating junctions which are present between the cells. Now these communicating junctions are basically the gap junctions very famously known as gap junctions. What happens is that there is presence of certain proteins on the membrane and these proteins are transmembrane proteins that is they travel the entire membrane. So these proteins are known as conixins. Now you see in this diagram here two membranes are shown okay. So maybe this is one cell right and this is another cell. So these membranes are shown here in this diagram and this particular protein is basically the conixin. Now there are six conixins you see 1, 2, 3, 4, 5, 6, 6 conixins which are present on the membrane. So here they are present like this. These six conixins form a pore so they are arranged like that they are forming a pore. Then these six conixins in turn interact with the six conixins which are present on the other membrane. So here there will be six conixins further which are forming a pore. By the way six conixins together form a conixon which is basically a hemichannel. So this one conixon forms a hemichannel and when this one conixon interacts with conixon on the other membrane this forms the complete channel or that is the gap junction. So the pores come close together right and the substances can travel through these pores from one cell to another. Very important example of this is in cardiac muscle. Cardiac muscle behaves as a syncytium. Why? Because the muscle cells are connected together by these gap junctions. So if there is depolarization in one muscle the ions from that muscle cell travel to other muscle cell and they cause depolarization there also and this travelling time is very fast. Okay so that is why the entire cardiac muscle basically the atria separately and the ventricles separately they act as a syncytium. That is they act as one unit. Now just one extra thing here that you see that these proteins that is the conixins they are transmembrane proteins as I told you they also have loop for regulation of the gap junctions that means the functioning of these gap junctions can also change. So we have discussed all three modalities of intracellular communication. More details of intracellular junctions will be covered in next lecture.